It is known that when colored substances are subjected to polychromatic radiation such as, for example, daylight, they have the property of reflecting, transmitting or scattering only certain wavelengths and of absorbing the remainder of the luminous energy, which is dissipated by nonradiative processes. So-called daylight-fluorescent substances have the additional property of converting a proportion of the radiation absorbed at the blue end of the visible spectrum and in the near UV into light which is reemitted at longer wavelengths, also situated in the visible spectrum and equal to those of the light which these substances do not absorb. Through this process, they are capable of producing in the observer's eye an impression of color and of brightness which is up to four times greater than that of ordinary colored substances of the same color. It is also known that the intensity of the emitted fluorescent light is extremely sensitive to so-called fluorescence extinction phenomena and that it is a function especially of the concentration of the fluorescent substance itself (autoextinction phenomenon) and of the possible presence of other substances known as fluorescence inhibitors (which act, for example, by reabsorbing the emitted light or by nonradiative quantum deexcitation processes).
In most applications of colorants (for example paper coating, textile printing, plastic coatings) the colorant molecules must be prevented from migrating, diffusing or redissolving in a solvent. In the case of fluorescent colorants, furthermore, the fluorescence intensity is at a maximum (low autoextinction) in a precise concentration range and, if the other causes of extinction of fluorescence are to be limited, the colorant matter must be protected in an inert but transparent optical medium. A rigid polymer matrix in which the colorant molecule is soluble (solid solution) or dispersible meets these requirements of isolation, confinement and immobilisation. These colored polymers are employed in most cases in the form of finely ground particles, generally referred to as pigments.
The polymers employed for manufacturing fluorescent pigments belong to the classes of thermoplastic and thermosetting resins. Among those most commonly employed are aminoplastic resins resulting from the polycondensation of triazines, amines and formaldehyde. Other polymers, such as polyesters, polyamides and polyurethanes and polyvinyl chlorides can also form the carrier for colorant molecules. Depending on the degree of crosslinking obtained during the polymerisation, these resins are either thermoset or thermoplastic. Thermoset resins are employed in cases where good resistance to solvents and to plasticisers is required (absence of swelling and of colorant diffusion) and when softening under the effect of heat could create problems.
In known processes for the manufacture of these thermoset resins the above mixture is polycondensed in bulk, in noncontinuous batches. Such processes are described e.g. in U.S. Pat. No. 3,939,093, in GB 1,341,602 or in U.S. Pat. No. 3,812,051. On the average the reaction takes 2 hours, per batch, in the reactor. After complete polymerisation a hard, tough solid is obtained, whose texture often resembles that of horn. This solid must be taken out of the polymerisation reactor as a block. This can prove difficult and it is often preferred to complete the reaction by casting the reacting mass, which is still pasty, into troughs and finishing the polymerisation in an oven. The blocks are then crushed and then micronised. The micronisation of this solid presents some difficulties: it requires a pregrinding before a fine microniser is fed, it being necessary for the two items of equipment to be cleaned after each batch. Such conventional processes for the manufacture are also described in Chem. Brit., 335 (1977). The U.S. Pat. No. 3,972,849 proposes the use of known grinding equipment, such as a ball mill, as the reaction vessel in an attempt to avoid the dissadvantages of the conventional manufacturing process.
The inconvenience of the conventional manufacturing processes and the disadvantages of the pigment particles obtained by these processes have led some manufactures, on the other hand, to prefer the manufacture and the use of pigments based on thermoplastic resins each time that a high solvent and temperature resistance is not essential. U.S. Pat. No. 2,809,954, GB 869,801 and GB 980,583 describe the synthesis of pigments based on thermoplastic resins. These fusible, and hence heat-sensitive, resins do not lend themselves well to simple micronising by milling and hence to the manufacture of pigments of a fine and well-determined particle size. These resins generally require an additional stage of manufacture (dispersion, phase separation) to obtain pigment particles of well-determined particle size, which is described, for example, in U.S. Pat. Nos. 3,642,650 and 3,412,034.
The disadvantages of the two types of processes described above are avoided in the manufacture of amide (urea, melamine, and the like)/formaldehyde condensates of low molecular weight or of polyester alkyd resins, wherein to each type of said resins the colorant is attached by affinity. Such processes are described for example in GB 748,848, GB 786,678 or in GB 733,856. However, the applications of such pigments are in practice limited to inks and paints, because the colorant molecules are bound to the condensates only by affinity.